1. Field of the Invention
The present invention relates to an apparatus for detecting the distance between one's car and the car ahead (the apparatus being hereinafter referred to as "car distance detecting apparatus"), which is designed so that a light projector and a light receiver which are mounted on one's car are automatically rotated so as to be aimed at a reflector mounted on the car ahead to project pulse light which has a specific code from the light projector onto the car ahead and receive the reflected light from the reflector by the light receiver, thereby detecting the distance between the one's car and the car ahead, together with the angle between the axes of the two cars, by measuring the angle of projection of the optical system.
2. Description of the Prior Art
An active distance measuring apparatus has heretofore been known which is arranged such that, with a light projector and a light receiver spaced apart from each other by a predetermined base line distance, pulse light is projected from the light projector toward an object of measurement, with the light receiver being rotated, and the angle of the light receiver when detecting the reflected light from the object is detected to thereby detect the distance to the object. Such a distance measuring apparatus has been utilized, for example, for the automatic focusing system of cameras.
The principle of the conventional distance measuring apparatus is shown in FIG. 6. Referring to FIG. 6, the reference numeral 11 denotes a light emitting element, for example, an infrared LED (light Emitting Diode), and 9 a light projecting lens, which constitute in combination a light projector.
The reference numeral 12 denotes a photoelectric transducer, for example, a photodiode or PSD (Position Sensitive Device), and 10 a light receiving lens, which constitute in combination a light receiver.
The reference numeral 19 denotes an object of measurement, while the reference symbol D denotes a predetermined base line distance, and L the distance to the object 19. Here, L can be obtained as being L=D.times.tan.theta..
In operation, the light emitted from the light emitting element 11 is condensed through the light projecting lens 9 to irradiate the object 19, and the reflected light from the object 19 is condensed through the light receiving lens 10 and received by the photoelectric transducer 12 where the distance L to the object 19 is obtained as being L=D.times.tan.theta..
The conventional distance measuring apparatus suffers, however, from the following problems. That is, the object 19 must be present on the optical axis of the light projector. In the case of a camera, the user can put the object 19 on the optical axis of the light projector by moving the camera himself. In the case where such a distance measuring apparatus is used to detect the distance between one's car and the car ahead, however, the driver must adjust the position of the light projector so that the car ahead is put on the optical axis of the light projector every time the distance between the two cars is desired to detect, which loads the driver so heavily that safe driving is disturbed.
When the light projector is fixed, on the other hand, detection of the distance is difficult and extremely low in terms of reliability in the case of a car distance detecting apparatus in which the change of the axis of one's car and that of the car ahead are not correlated to each other.